ReviewAB5 toxins: structures and inhibitor design
Introduction
AB5 toxins are a class of medically important bacterial toxins, named after their unique architecture comprising a single catalytically active A subunit and a pentamer of B subunits. The B pentamer is responsible for binding to cell-surface receptors, a function retained even in the absence of the A subunit. However, the complete AB5 holotoxin is required for the toxic effects. The class of AB5 toxins may be subdivided into families on the basis of sequence homology and catalytic activity (Fig. 1). The cholera toxin (CT) family includes CT itself, the Escherichia coli heat-labile enterotoxins (LTs) LT-I [1], [2], [3] and LT-II [4], [5], [6], and a less well characterized toxin from Campylobacter jejuni [7]. These toxins all have A subunits with ADP-ribosylation activities targeting an arginine of Gsα (the α subunit of the stimulatory trimeric G protein). CT and LT-I share 80% sequence homology in both the A and the B subunits, whereas they have lower sequence homology to LT-II. The Shiga toxin (SHT) family [8] includes a number of toxins from Shigella dysenteriae [9] and the Shiga-like toxins (SLTs) (also known as verotoxins) from certain E. coli strains (SLT-I and SLT-II) [10]. SHT and SLT-I are almost identical, with small variations in their A subunits, which are N-glycosidases. SHT and SLT-I are about 60% identical to SLT-II in both the A and the B subunits. Pertussis toxin (PT), produced by Bordetella pertussis and the causative agent of whooping cough, has very low sequence homology to the CT and SHT families of AB5 toxins, yet structural homology is preserved [11]. It is an exceptional AB5 toxin in that its five B subunits are not identical. The A subunit of PT catalyzes the ADP-ribosylation of a cysteine of Giα (the α subunit of the inhibitory trimeric G protein). The AB5 toxins have a wide range of toxic effects on human populations, from the relatively mild travelers’ diarrhea caused by LT-I to the much more serious and sometimes life-threatening diarrhea caused by Vibrio cholerae and the hemolytic uremic syndrome (‘hamburger disease’) caused by members of the SHT family. The AB5 toxins are probably responsible for over a million deaths annually worldwide and remain a severe medical problem.
The structure and function of the AB5 toxins were reviewed in detail in 1995 [12]. Since then, a large collection of structures of these toxins in a variety of forms has been obtained. These structures provide new insights into the biological functions of the toxins, as exemplified by the recently determined structure of a group II SLT mutant in complex with a receptor analog, which revealed residues critical for receptor binding specificity [13]. In addition, high-resolution structures of AB5 toxins with or without a bound ligand offer critical guidance for the structure-based design of inhibitors that can potentially be used to combat AB5-toxin-caused diseases. The great advances since 1999 in ligand design will be the main focus of this brief review.
Section snippets
New structural insights: receptor binding by Shiga-like toxins
Although members of the cholera family of toxins each contain five identical receptor-binding sites, each formed primarily by one B subunit with a minor contribution from a neighboring subunit, the SLTs are more complicated (Fig. 2). Despite the smaller size of their B subunits, SLT-I and SLT-II are found to bind gangliosides at up to three sites per B subunit. Specifically, the crystal structure of the SLT-I B pentamer in complex with a trisaccharide analog of SLT-I's natural receptor, Gb3,
Structure-based inhibitor design for AB5 toxins
So far, more than 30 high-resolution crystal structures of AB5 toxins are available, mostly for the cholera and the Shiga families of toxins, in a wide variety of apo, mutant and complex forms. They form an excellent basis for designing potent inhibitors against these toxins through a structure-based approach [17]. The mechanism of action of AB5 toxins offers several critical steps that can be targeted. First, one could design inhibitors that block the assembly of the holotoxin. Second, one
Conclusions
Many crystal structures of AB5 holotoxins or their subunits in complex with the natural receptors or synthetic inhibitors have appeared in the past few years. Such high-resolution structures provided critical insights for the design of novel toxin inhibitors through structure-based drug design and combinatorial chemistry. Particular progress has been made since early 1999 towards designing inhibitors blocking either the toxin assembly process or the cell-surface receptor recognition process.
Acknowledgements
The authors would like to thank Professor David Bundle and Bianca Hovey for providing figures, and the National Institutes of Health for financial support (AI44954 to EF, GM54618 to CLMJV and AI34501 to WGJH).
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
of special interest
of outstanding interest
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